By bicycle gearshift a mechanical device is meant, that causes the transmission chain to move between different toothed wheels, for this purpose moving a chain guide in which the chain is engaged.
In the context of this patent description and of the following claims, the gearshift being referred to is the rear one that moves the transmission chain between the different sprockets of a cogset associated with the rear wheel of the bicycle.
Normally, the bicycle gearshift comprises a kinematic mechanism in the form of a four-bar linkage (typically an articulated parallelogram) with a base body and a mobile body opposite the base body in the four-bar linkage, connected together through a pair of connecting rods hinged to such bodies according to four hinge axes through four pin elements, in which the base body is fixed to the frame of the bicycle and the mobile body is fixed to a chain guide.
The deformation of the four-bar linkage therefore determines a displacement of the chain guide with respect to the frame in the axial direction with respect to the cogset and, in this way, gearshifting.
The deformation of the four-bar linkage can be obtained with manual actuation, through the movement of control levers and the transmission thereof to the four-bar linkage through a Bowden cable, or with motorized actuation, thanks to an electric motor that—after a suitable command imparted by the cyclist and through a suitable mechanism—moves different parts of the four-bar linkage with respect to one another, thus deforming it and moving the chain guide.
Bicycle gearshifts with motorized actuation are described for example in EP1357023; in them, the kinematic mechanism acts by moving opposite pins of the four-bar linkage towards and away from one another.
An ongoing objective of manufacturers of gearshifts is to improve the precision of actuation, upon which the ease and reliability of operation of the gearshift depends.
This requirement is increasingly important the more the gearshift is intended to be used in high-level cycling competitions.
In order to obtain high precision of control of known bicycle gearshifts, an initial adjustment of the bicycle is made in order to optimise the tensioning of the chain depending on the configuration and structure of the frame and of the cogset associated with the rear wheel.
The initial adjustment acts on at least one spring inserted in the kinematic mechanism, possibly applying a preload thereto, in order to keep the correct tension of the transmission chain in the different travel configurations.
In some gearshifts used particularly for road bicycles, two chain tensioning springs are inserted into the kinematic mechanism, said springs working in antagonism to determine the set-up of the chain guide when engaged with the chain of a transmission. This allows greater versatility of the system, giving it high elasticity. Other gearshifts, on the other hand, can foresee a single chain tensioning spring.
The initial setting of the preload of the chain tensioning springs is carried out so as to raise the chain guide to bring it as close as possible to the sprockets.
Indeed, a small distance between the chain guide and the sprockets determines greater sensitivity of control since, in such conditions, the displacement component of the chain guide parallel to the axis of the sprockets corresponds to an inclination exerted on the chain that is sufficient to trigger a displacement from one sprocket to the other.
The lifting of the chain guide towards the sprockets does, however, have a limitation dictated by the sprocket of largest diameter. Moreover, getting too close to the sprocket with largest diameter is the cause of drawbacks such as the feeling of sudden gearshifting between the lowest gears to the next one, as well as slipping between the chain and the chain guide in the case of the lowest gear and pedalling backwards.
The Applicant has realised that in the adjustment configuration closest to the sprockets, set by the dimensions of the largest sprocket, there is still a substantial vertical spacing between the chain guide and the sprockets with smaller size and, consequently, the precision of gearshifting control between the highest gears is less than between the lower gears.
Therefore, the problem forming the basis of the invention is that of avoiding the aforementioned drawbacks, in particular by providing a bicycle gearshift that is able to offer improved precision gearshifting.
More specifically, the problem forming the basis of the present invention is that of making a bicycle gearshift that allows the distance of positioning between the chain guide and the smaller sized sprockets to be reduced with respect to known gearshifts, without however entailing the drawbacks dictated by bringing the chain guide too close to the largest sprocket.